After much consideration I replaced the battery in my AT with a Hawker SBS8 battery.
The reason why I hessitated for so long was the modifications it involved, but afterall this was not a problem.
The + and - are reversed, but this is easely corrected by placing the battery backwards.
Now you drill a 4 mm hole in the original battery-clambs, and they can be fitted. You need to cut out a little bit of plastic on the (-) side to make the cable go in easyer, but that's all.
To "fasten" the smaller battery I attached a L-shaped piece of aluminum on the inside of the compartment-cover pressing ontop of the battery (with a sheet of rubber).
So... it's easy and reversable.